1. Field
The present invention relates generally to circuits, and more specifically to techniques for fabricating analog and digital circuits on separate dies and stacking and integrating the dies into a single package.
2. Background
Many applications require both analog and digital signal processing. One such application is in the area of wireless communication, where mixed analog and digital signal processing is required on both the transmit and receive sides. On the receive side, a modulated analog signal (typically at radio frequency) is received, conditioned (e.g., amplified and filtered), downconverted, quadrature demodulated, and digitized to provide samples. Digital signal processing is then performed on the samples to recover the transmitted data. And on the transmit side, data is processed (e.g., encoded, interleaved, and spread) and then converted to one or more analog signals. The analog signals are then conditioned, modulated, and upconverted to provide a modulated signal suitable for transmission over a wireless link. Mixed signal circuits may also be used for other aspects of wireless communication, and include voice/audio coder/decoder, general-purpose analog-to-digital converters (ADCs) to digitize various signals such as battery voltage and temperature, and other circuits. Mixed-signal processing is also required for many other applications such as networking, computer, and others.
Conventionally, analog and digital signal processing is achieved via separate analog and digital integrated circuits (ICs), with the interface between the two ICs achieved via ADCs and digital-to-analog converters (DACs). Digital circuits tend to generate large amounts of switching noise. In contrast, analog circuits typically include various sensitive circuits (e.g., oscillator, amplifier, and so on) that prefer, or are required, to operate in a quite environment. Implementation of analog and digital circuits on separate ICs allows these circuits to be isolated and operated in their preferred environments. Moreover, the optimum process technologies for analog and digital circuits are typically different. Digital circuits are often implemented using standard CMOS process, whereas analog circuits may utilize linear capacitors and resistors that require extra processing steps to be added to the standard CMOS process.
To reduce cost and complexity of a product, both analog and digital circuits may be fabricated on a common substrate in a mixed-signal IC. The mixed-signal IC provides numerous benefits such as reduced cost, fewer component count, smaller required board area, simplified testing, and possibly other benefits.
However, the fabrication of the analog and digital circuits on a common substrate has some disadvantages. First, the noise generated by the digital circuits degrades the performance of the analog circuits via coupling through the substrate. Second, the analog circuits may require linear capacitors and resistors, which may then dictate the need for a particular IC process, such as analog CMOS. Thus, although the analog circuits may occupy only a small portion of the die, the cost of the digital circuits increases as a consequence of the IC process selected for the analog circuits. Third, digital circuits typically benefit from technology scaling (e.g., reduction in transistor sizes, lower operating voltages) whereas analog circuits may suffer from voltage scaling. And fourth, the design cycle of a mixed-signal IC may be prolonged because the design cycle for analog circuits is typically much longer than that for digital circuits.
As can be seen, there is a need in the art for techniques to fabricate and integrate analog and digital circuits such that the benefits of mixed-signal ICs can be attained while the disadvantages of conventional mixed-signal ICs fabricated on a common substrate are minimized.
Aspects of the invention provide techniques for fabricating analog and digital circuits on separate dies and stacking and integrating the dies within a single package to form a mixed-signal IC that provides many of the above-described benefits. In one aspect, the analog and digital circuits are implemented on two separate dies using possibly different IC processes suitable for these different types of circuits. The analog and digital dies are thereafter integrated (stacked) and encapsulated within the single package. Bonding pads are provided to interconnect the two dies and to connect the dies to external pins. The bonding pads may be located and arranged in a manner to provide the required connectivity while minimizing the amount of die area required to implement the pads. In another aspect, the die-to-die connectivity may be tested in conjunction with a serial bus interface. In yet another aspect, the power supplies provided to some or all of the circuits within the analog die (and possibly the power supplies provided to some or all blocks in the digital die) may be collapsed (e.g., brought down to zero Volt) during a stand-by mode to extend operating.